Formation of excited uranyl ion in oxidation of U(IV) with xenon trioxide in aqueous solutions of H2SO4 and HClO4: I. Influence of sulfate anions

Article abstract of DOI:10.1023/B:RACH.0000024937.45748.1c

Kinetic and chemiluminescent parameters of U(IV) oxidation with xenon trioxide in 1 M H₂SO₄ were studied in relation to complexation of U(IV) with SO₄^2- anions. The rate constant of oxidation of the sulfate comp

Full text of DOI:10.1023/B:RACH.0000024937.45748.1c

Radiochemistry, Vol. 46, No. 2, 2004, pp. 131 135. Translated from Radiokhimiya, Vol. 46, No. 2, 2004, pp. 119 122.
Original Russian Text Copyright 2004 by Khamidullina, Lotnik, Kazakov.
Formation of Excited Uranyl Ion in Oxidation of U(IV)
with Xenon Trioxide in Aqueous Solutions of H SO and HClO :
2
4
4
I. Influence of Sulfate Anions
L. A. Khamidullina, S. V. Lotnik, and V. P. Kazakov
Institute of Organic Chemistry, Ufa Research Center, Russian Academy of Sciences,
Ufa, Bashkortostan, Russia
Received February 11, 2003
Abstract Kinetic and chemiluminescent parameters of U(IV) oxidation with xenon trioxide in 1 M H SO
2
4
2
were studied in relation to complexation of U(IV) with SO anions. The rate constant of oxidation of the
4
1
1
3
sulfate complexes increases from 1 to 43 l mol
3
s
with increasing [Na SO ] in the solution from 10 to
10 M. The equilibrium concentrations of aqua- and mono- and bisulfate complexes of U(IV) were
2 4
1
calculated at different [Na SO ] in the solution. The rate constant of reaction of U(SO ) with XeO [k =
2
4
4 2
3
1
7
2 l mol 1 s ] is higher by an order of magnitude than the rate constant of oxidation of U(IV) monosulfate
1
1
complex [k = 6.5 l mol s ]. The dependence of the lifetime of excited uranyl ions ( ) on the Na SO con-
2
4
centration in 1 M HClO was measured. The chemiluminescence (CL) yield increases in the presence of
4
sulfate anions owing to the growth of both the emission output of excited uranyl sulfate complexes and the
yield of excitation of the CL emitter.
Xenon trioxide is a strong oxidizing agent. The
from them were described in our previous work [7].
An examined solution was placed in a glass reactor
where it could be vigorously stirred and its tempera-
ture could be maintained with an accuracy of 0.1 K.
The solution temperature was permanently measured
with a copper Constantan thermocouple sealed in
potential of the XeO /Xe couple in an acidic solution
3
is 2.12 [1], i.e., it is close to that of ozone. Bright
luminescence of U(IV) oxidation with xenon trioxide
in perchloric and sulfuric acid solutions allows the
kinetics of this reaction to be studied at the reactant
concentrations lower by several orders of magnitude
than those used in the spectrophotometric procedure
2+
a thin-walled glass capillary. The lifetime of (UO )*
2
was measured on a -metric unit consisting of an
[
2 4]. We found that the rate of the chemiluminescent
IGL-300/2 nitrogen laser (
= 337.1 nm, pulse time
exc
reaction depends on the H SO concentration. Hydro-
8
2
4
10 s); a -measuring block giving digital value of
time in which the luminescence decreases by a factor
of e after a single photoexcitation pulse (the error
of a single measurement of is lower than 5%); and
a cryostat maintaining the sample temperature in the
range 77 300 K with an accuracy of 1 K. A solution
to be studied (0.2 0.5 ml) was placed in a quartz
test tube 5 mm in diameter, the measuring junction
of a thermocouple in a glass capillary was inserted in
the solution, and the tube was fixed in a quartz Dewar
vessel purged with air or a vapor of boiling nitro-
gen. Uranyl photoluminescence (PL) passed through
crossed light filters was registered at a right angle to
the excitation pulse with an FEU-119 detector.
gen ions inhibit and sulfate ions catalyze the reaction
of U(IV) with XeO . In this work, we studied the
3
kinetics and chemiluminescent parameters of U(IV)
oxidation with xenon trioxide in perchloric and sul-
furic acid solutions as influenced by complexation of
U(IV) with SO2 anions. Since ClO anions do not
4
4
coordinate to U(IV) [5], we used 1 M HClO as the
4
solvent in the blank experiment.
EXPERIMENTAL
Sulfuric acid and water were distilled until the
absorption at > 200 nm in a 5-cm cell disappeared.
3
1
The UO SO solutions (10 10 M) were prepared
2
4
by dissolution in H SO4 of UO3 prepared from
UO (NO ) 6H O of chemically pure grade [6]. The
2
RESULTS AND DISCUSSION
2
3 2
2
equipment used for recording the chemiluminescence
CL) and the procedures for preparing and purifying
the required chemicals and for preparing solutions
(
When the oxidizing agent is present in the solution
in an excess with respect to U(IV), the reaction is
1
066-3622/04/4602-0131 2004 MAIK Nauka/Interperiodica

132
KHAMIDULLINA et al.
tion at constant concentration of sulfate ions ( 1 M)
and constant ionic strength of the solution ( = 1.5).
In this case, the reaction decelerated with increasing
+
+
[
H ]. The slope of the logk vs. log[H ] dependence is
1
.3 (Fig. 1, curve 2), i.e., the reaction order with
respect to protons is 1.3. Hence, deceleration of
the reaction with increasing H SO concentration is
2
4
caused by the fact that the inhibiting action of protons
prevails over catalytic actions of sulfate anions. Thus,
the rate constant of the reaction can be described by
the equation
d[U(IV)]/dt = k [U(IV)][XeO ]/[H ]1.3.
+
(3)
0
3
Fig. 1. Rate constant of U(IV) oxidation with xenon triox-
To explain inhibition with protons of U(IV) oxida-
+
ide as a function of concentration of (1) H SO and (2) H .
2
4
tion with oxygen in aqueous HClO , Sobkowski [8]
4
4
6
[
XeO ] = 10 , [U(IV)] = 2 10 M; T = 293 K.
3+
3
0
0
assumed that the U(IV) hydrolysis products, U(OH)
2+
and U(OH) , were involved in the reaction. The equi-
librium constants of formation of these species in
2
described by a first-order kinetic equation
HClO at
= 1 by the reactions
U4+ + H O U(OH)3+ + H+,
U(OH) 22 + + 2H+
4
I = V = k[U(IV)][XeO3].
(1)
(4)
(5)
2
The stoichiometric equation of the reaction is as
follows:
U4+ + 2H O
2
UO + Xe + 6H+
2
+
2
2
are K = 2.7 10 2 and K = 1.6 10 4 [9, 10]. How-
4
5
3
U4+ + XeO + 3H O
(2)
3
2
ever, in our case reaction (2) is performed at an excess
2
2
+
UO 22 + + h .
of sulfate anions, i.e., [U(IV)] << ([HSO ] + [SO ]) =
*
(UO )
4 4
2
1
. Under these conditions, sulfate complexes of U(IV)
are formed by the reactions
The spectrum of CL passed through the glass light
filters with sharp cutoff is similar to that of uranyl ion.
The rate constants measured in 1 M H SO at 275.5,
U4+ + SO2
2+
USO ,
4
(6)
(7)
4
2
4
1
1
2
93, and 315 K are 7.8, 33.7, and 157.2 l mol s ,
respectively. The CL kinetics in the temperature range
15 275.5 K is described by the Arrhenius equation,
with the activation energy of reaction (2) being
USO2 + SO2
+
U(SO ) ,
4
4
4 2
3
Since the equilibrium constants of these reactions
1
1
(
K = 330 and K = 22 l mol [11]) are much higher
1
3.5 kcal mol . The chemiluminescence efficiency
6 7
6
6
than K and K , the major portion of U(IV) is in the
( CL) at 275.5, 293, and 315 K is 1 10 , 5.5 10 ,
and 8.7 10 , respectively. The appreciable differ-
4 5
6
form of sulfate complexes and the equilibrium con-
centrations of U(OH)3 and U(OH)2 are so low that
participation of these species in U(IV) oxidation with
xenon trioxide can be neglected.
+
2+
ence in
dependence of the radiation output of the CL emitter
( rad). The parameter is proportional to the life-
time of electronically excited uranyl ion ( ), which is
equal to 45, 16, and 5 s in 1 M H SO at 275.5, 293,
is mainly due to the strong temperature
CL
rad
We suggest that hydrogen ions change the second
coordination sphere of U(IV) sulfate complexes, thus
inhibiting reaction (2). Uranium(IV) complexes in
which the number of sulfate ligands is smaller than
4 usually contain water molecules. In our case, sul-
2
4
and 315 K, respectively.
The rate constant decreases with increasing H SO4
2
concentration (Fig. 1, curve 1). To study this con-
centration effect, we performed two series of experi-
ments. First, we varied the concentration of sulfate
anions at constant proton concentration and constant
ionic strength of the solution. In this case, the reaction
was accelerated with increasing concentration of sul-
fate anions. Second, we varied the proton concentra-
2
+
fate aqua complexes [USO (H O) ] and [U(SO4)2
4
2
6
(H O) ] should be formed [12]. The bond between
2
4
U(IV) and a sulfate ligand is weak and the composi-
tion of U(IV) sulfate complexes depends on the total
concentration of the solution components, excess or
deficiency of sulfate anions, pH, etc. Since the second
RADIOCHEMISTRY Vol. 46 No. 2 2004

FORMATION OF EXCITED URANYL ION IN OXIDATION OF U(IV)... : I.
dissociation constant of H SO is low (K = 1.2
133
2
4
d
2
1
0
[13]), free sulfate anions are converted into bisul-
fate anion with increasing acidity of the solution.
Since the formation constants of U(IV) complexes
with bisulfate anions are substantially lower than
those with sulfate anions [12], in acidic solutions the
sulfate ligands should be substituted with water
molecules present in a large excess:
+
2+
[
U(SO ) (H O) ] + H + 2H O
[USO (H O) ]
4 2 6
4
2
2
4
2
+
HSO4.
(8)
We will show below that the neutral complex
2+
Fig. 2. Kinetics of chemiluminescence of U(IV) oxidation
with XeO at constant acidity (1 M HClO ) and ionic
U(SO ) is oxidized much faster than USO . Hence,
4
2
4
the rate constant of U(IV) oxidation with xenon triox-
3
4
+
strength of the solution ( = 1.5) and Na SO concentration
2 4
ide decreases with increasing [H ] owing to partial
2
2
1
of (1) 0, (2) 2.5 10 , (3) 7.5 10 , and (4) 3 10 M.
[
dissociation of the neutral U(IV) complex.
3
4
U(IV)] = 5 10 , [XeO ] = 10 M; T = 293 K.
0 3
We studied the influence of SO2 anions on the
4
trioxide; and k , rate constant of oxidation of U4
+
kinetics of chemiluminescent oxidation of U(IV) with
XeO in 1 M HClO . The kinetic curve of CL re-
ox
aq
with xenon trioxide. Since the CL yield in the first
reaction is higher, the CL intensity is described by the
equation
3
4
corded after addition of a U(IV) solution to a sodium
sulfate solution containing XeO passes through a
3
maximum (Fig. 2, curves 3, 4). It should be noted the
CL intensity increases with increasing concentration
of sulfate anions. This is due to both acceleration of
oxidation of the U(IV) complex cation as compared to
oxidation of the aqua complex and an increase in the
CL efficiency.
2
+
I =
k [XeO ][USO ].
(10)
CL 2
3
4
The sulfate complex USO2 is an intermediate
of consecutive pseudo-first-order reactions, since
+
4
[
U(IV)]0 < [XeO ] and [U(IV)] << [HSO ]. The
3 0 4
concentration of this complex at time t is determined
If the sequence of reactant mixing changes, the
maximum in the kinetic curve disappears. After addi-
by the equation [14]
2
+
k t
1
k2t)}. (11)
e
tion of XeO to a solution containing U(IV) and sul-
[USO ] = {k [U(IV)] }/{(k
2
k1)(e
3
4
1
0
fate anions, the CL intensity monotonically decreased
owing to oxidation of the U(IV) sulfate complex with
XeO . Since the reaction is performed in 1 M HClO ,
Substitution of Eq. (11) in Eq. (10) gives
3
4
k t
k2t)}. (12)
e
the second step of H SO dissociation is shifted to
I =
k [XeO ]k [U(IV)] /{(k
2
k1)(e
1
2
4
CL 2
3
1
0
2
formation of bisulfate anion and [SO ]/[HSO ]
4
4
2
1
0 .
Equation (12) contains three unknowns: CL yield,
k , and k . Using the ratio of the CL intensity at time t
and at the maximum in the chemiluminescent curve
1
2
We suggest that the maximum of the CL intensity
in the kinetic curve is caused by the following reac-
tions:
(
I/Imax), we obtain the equation for the relative intens-
ity and exclude
:
k
k
2
1
k t
k t
k t
1 max
k2tmax), (13)
e
2
+
1
2
Irel = I/Imax = (e
e
)/(e
USO
*(UO SO )
UO SO + h
2 4
4
2
4
1
+
HSO
+XeO
3
4
where tmax is the time required to reach the maximum.
Equation (13) was derived on an assumption that
4
+
U
(9)
aq
k1 >> k and the reaction occurs mainly by the first
k
ox
ox
UO+2
2+
UO 22 + + h 2,
*(UO )
2
pathway. The calculated dependence of the relative
intensity of CL (Irel = I/Imax) on time fits well the ex-
perimental data (Fig. 3, curves 2 and 1, respectively).
+
XeO
+ XeO
3
3
2
+
where k is the rate constant of formation of USO ;
1
4
2+
k2, rate constant of oxidation of USO4 with xenon
The calculated rate constant of substitution of water
RADIOCHEMISTRY Vol. 46 No. 2 2004

134
KHAMIDULLINA et al.
tion from 10 3 to 3 10 M. (Fig. 4, curve 1). Using
the known [15] equilibrium constants of formation of
U(IV) sulfate complexes
1
U4+ + HSO4
U4+ + 2HSO4
U(SO4)2+ + H+,
U(SO4)2 + 2H+,
(14)
(15)
K14 = 130 and K15 = 1380, we calculated the equi-
librium concentrations of aqua, mono-, and bisulfate
complexes of U(IV) at different concentrations of
sulfate anions in the solution (Fig. 5, 1 3).
Fig. 3. Relative intensity (I
= I/I
) of chemilumi-
max
rel
nescence in the course of U(IV) oxidation with XeO in the
3
2
presence of SO in the solution as a function of time.
4
Representation of the U(IV) concentration as a sum
of the concentrations of the aqua-, mono-, and bisul-
fate complexes and of the observed rate constant of
Points are experimental, and the continuous line is calcu-
lated.
U(IV) oxidation with XeO as a sum of the rate con-
3
stant of oxidation of the corresponding complexes
gives a system of equations. The rate constants of
oxidation of mono- and bisulfate complexes of U(IV)
with XeO , calculated from this system, are 6.5 and
3
1
1
7
2 l mol s , respectively. Thus, acceleration of
U(IV) oxidation with XeO with increasing Na SO
3
2
4
concentration is due to formation of U(SO ) com-
4
2
plexes which are oxidized with the rate constant
higher by an order of magnitude compared to oxida-
2
+
tion of USO .
4
The CL yield ( CL) described by Eq. (10) depends
on the Na SO concentration in the solution. We sug-
2
4
gest that
increases owing to an increase in the
CL
2+
Fig. 4. (1) Rate constant of U(IV) oxidation with XeO and
3
emission output of excited uranyl ions *(UO ), i.e.,
2
(
2) lifetime ( ) of excited uranyl ions in 1 M HClO as
4
coordination of sulfate anions to uranyl decreases the
probability of nonradiating deactivation. This is pos-
sible only if in the course of U(IV) oxidation passing
5
functions of the Na SO concentration. [U(IV)] = 5 10
XeO ] = 10 , and [UO ] = 10 M; T = 293 K; and
,
2
4
0
4
2+
3
[
3
1.5.
2
=
+
through UO formation the sulfate groups will remain
2
coordinated to uranyl ion at least for the time of pho-
ton emission (10 4 10 s). However, the observed
5
increase in
is caused not only by the dependence
CL
of on the sulfate ion concentration (Fig. 4, curve 2)
but also by the influence of UO sulfate complexes on
the yield of *(UO ), since
+
2
2
+
=
exc.
2
CL
em
ACKNOWLEDGMENTS
This work was financially supported by the Rus-
sian Foundation for Basic Research (project no. 02-
03-32406).
4
+
2+
Fig. 5. Calculated content of (1) U , (2) USO , and
aq
4
(
3) U(SO ) as a function of the Na SO concentration in
4 2 2 4
5
1
M HClO . [U(IV)] = 5 10 M; T = 293 K.
4 0
molecules with sulfate anions in the coordination
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2
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